This invention relates to a temperature control apparatus and method and more particularly to a temperature control apparatus and method for use in connection with multi-section, interactive systems which have a large thermal mass and in which the temperature sensing devices are located sufficiently far from the source of heat that a significant time lag occurs between the time heat is applied and the time when it is first recorded or sensed by the sensing device.
Temperature control apparatus and methods are well known. They generally require one or more temperature sensors whose output is delivered to an electrical control circuit which compares the output of the sensor to a predetermined standard. As a result of the comparison, more or less power is applied to a temperature element associated with the temperature sensor. Several different types of temperature control circuits and sensors are commercially available for use in a variety of applications.
In those situations where temperature control apparatus is used to control the temperatures at at least two different locations in the same system, several problems may arise. First, the electrical controllers, where there is at least one controller associated with each location, may interact. Thus an increase in the heating power at one location in the system may affect the performance of a controller associated with another location in the system. While this introduces additional complexity into the temperature control apparatus, in most circumstances, there is commercially available apparatus to maintain the proper temperature control and the multiple controller system will maintain the proper temperature distribution and control.
In certain instances, however, commercially available systems are unsatisfactory. One example is temperature control in a high pressure vessel used to grow high quality quartz crystals. In the particular quartz growing process of interest, a pressure vessel is divided into upper and lower sections. A saturated quartz solution is maintained in the lower section and a seed crystal onto which quartz will deposit is placed in a quartz solution in the upper section of the vessel. The saturated quartz solution moves, by convection, into the upper section, where the solution becomes supersaturated which causes quartz to deposit onto the seed crystal. The two most critical temperature parameters affecting the growth of high quality crystals are the temperature in the top section of the vessel (the growth zone) and the difference in temperature between the top and bottom sections of the vessel. Since, the growing process provides a significant internal transfer of heat from the bottom section to the top section as the saturated fluid moves upward, there results what for commercially available control systems is an intolerable amount of interaction between the controller affecting the temperature in the top section and the controller affecting the temperature in the bottom section.
An additional problem in the quartz growing process is the extremely slow response of the thermodynamic system. This is especially true when the temperature sensors are placed in the fluid along the center line of the vessel, the preferred location to measure temperature since these are the actual temperatures which affect the process. The cycle time of such a system, depending upon the vessel construction, may be up to 24 or 48 hours. Under these circumstances, the time required to reach stable equilibrium, using commercial systems without manual intervention, is unacceptable since the resulting crystal is not of sufficiently high quality.
The resulting crystal is unacceptable because crystal from solution deposits continually while the temperature controller attempts to effect equilibrium. Thus as the temperature varies, so does the quality and structure of the crystal. To solve the problem of long cycle times, typically, the gain of the temperature controller is reduced to a value corresponding to the long cycle time or thermal time lag. This is a function of both sensor position and thermal mass. In addition to reducing the controller gain, these controllers often have reset capability which, applicants have found, further increases the control problem. Necessarily, manual intervention is then required to readjust the controller in order to reduce the time required to obtain equilibrium.
It is therefore an object of the invention to provide, in a temperature control system in which the temperature controllers interact and in which the thermodynamic response is slow, a temperature control method and apparatus which automatically achieves equilibrium conditions in a minimum time. Other objects of the invention are to provide an improved and superior temperature control method and apparatus which is simple in construction, efficient in operation, reliable, free from manual intervention, and which minimizes interaction between the controllers.